Drop SVGStylable as unimplemented

The SVGStylable interface as such has had no shipping implementation for
years. The current BCD data indicates it was previously in Firefox but
removed years ago, and was (maybe) part of (pre-Blink) Edge. It was
also apparently once part of Webkit, but in 2013 was removed in
https://trac.webkit.org/changeset/140265/webkit, with change message:

> SVG2 changes element inheritance so all SVGElements are stylable. As
> a first-step towards a cleaner class hierarchy, this patch moves
> SVGStylable into SVGStyledElement.

https://developer.mozilla.org/en-US/docs/Web/API/SVGStylable already has
it marked as deprecated.

Related: mdn#6768